Civil Engineering Department, Indian Institute of Technology, Kharagpur.
Direct measurement of drag forces in channels with laterally varying wall roughness with 3-dijferent arrangement of roughness concentration has been presented. A simple theoretical experssion connecting the ratio of drag forces over the smooth and total portion of the perimeter has been derived, on the basis of uniform shear distribution over the rough and smooth portion. From the plot of the ratio of measured drag forces, the ratio of shear stresses has been found to vary from 1.49 to 3.09 for all the 60 test runs (the theoretical formulation indicates a constant ratio of 2.60). Further the measured drag forces have been converted into friction. factors. These values have been compared with the computed friction factors obtained by synthesis from the resistance laws with homogeneous roughness distribution for the same flow depths by assigning weighfage properties al to perimeter. The computed friction factor s lie within ± 10 per cent of the observed values.
